Cut beef across the grain into thin slices about 2 inches (5 cm) long and 1/8 inch (3 mm) thick.
In small bowl, combine beef, vinegar, soy sauce, gingerroot and garlic; cover and marinate at room temperature for 30 minutes.
In wok or large skillet, heat 1 tbsp (15 mL) of the oil over high heat; stir-fry beef mixture for about 2 minutes or until browned.
Remove beef and juices from wok; reserve.
Heat remaining oil in wok; stir-fry onion and green and red peppers for about 2 minutes or until tender-crisp.
Return beef and juices to wok.
Stir in oyster sauce and sesame oil.
Dissolve cornstarch in 1 tbsp (15 mL) of the stock.
Stir remaining stock and cornstarch mixture into pan; cook for about 1 minute or until slightly thickened, stirring constantly.